Permanent magnet alloy



Feb. 20,

1 G. A. KELSALL ET AL PERMANENT MAGNET ALLOY Filed April 9, 1938 3 Sheets-Sheet VARIATION WITH PER CENT VANADIUM F REMANENGE,COERCIVE FORCE AND PRODUCT Ba x m FOR EQUAL PROPORTIONS of mo: AND CGBALT 26 COERCIVE FORCE ,moo U) I REMANENCE Q HEAT TREATMENT- quzucnso if FROM |30oc.oR |200c O FOLLOWED BY BAKING AT 000 c. Q: 2:0000 '-u 20 u D g g 100 80000 8 L6 2 Q; Q E 140 PRODUCT BexHc LI 5 6000 S Li E Q: U q Q S I00 Q 2 4 s a 12 14 no PER CENT VANAD/UM VARIATION WITH PER CENT COBALT OF REMANENCE, 260 COERCIVE FORCE m0 PRODUCT'B'; x H: FOR 8 IRON COBALT ALLOYS WITH lqgvmmnum Lu |o0 {7 400 240 Q: g 440 PRODUCT BR H 0 210000 3 400 200? a 360 REMANENcq Q a Q; 7

O U L6 8000 k 320 I60 I 2 g 280 g u t HEAT TREATMENT- quaucnso E FROM 1ao0c0n |2ooc. FOLLOWED E Q BY ammo AT e0ocv 4000 I60 COERCIVE FORCE PER cs/vr COB/1L7 GA. KELSALL MENU "5 ,4 NESB/TT 4 TTORNEV Feb. 20, 1940. KELSALL ET AL 2,190,667

PERMANENT MAGNET ALLOY Filed April 9, 1938 5 Sheets-Sheet 5 FIG. 5

s v 8 e 9 8 6? PER CENT COBALT aA/(ELsALL INVENTORS'EANESB/TT BYQWQKM ATTORNEY Patented Feb. 20, 1940 2,190,667

UNlTED STATES PATENT OFFICE PERMANENT MAGNET ALLOY George A. Kelsall, Beilevillc, N. J., and Ethan A.

Nesbitt, Brooklyn, N. Y., assignors to Bali Tolephonc Laboratories, Incorporated, New York, N. Y., a corporation New York 7 Application April 9, 1938, Serial No. 201,058

Claims. (01. 148-18) The invention relates to iron and cobalt alloys discovered contains approximately 38 per cent for use as strong permanent magnets and methiron, 52 per cent cobalt and 10 per cent vanadium ods of producing the same. and has a remanence of 9000 gauss, a coercive An object of the invention is to achieve a perforce of 300 oersteds and an energy product 5 manent magnet composition which possesses in (BXH) oi! l.02X Its heat treatment is not a high degree the comblnation'oi properties, first, critical. For optimum permanent magnet propability to ifunction as a permanent magnet equal erties, it is not necessary to use temperatures or better than the best materialsheretoforeavailabove 800 C. In other words, it may be subable in the art and, second, workability to a de' iected to either one or two heat treatments and 10 gree previously unattained in a permanent magafter either treatment, it may be quenched 10 net material in the first class as a permanent p dly s o y t o f c n its P ent magnet magnet properties. A subsequent baking treat- In order to achieve the main object of the inment at 600 to 800 C. is an essential feature of vention, that is, to produce permanent magnet the heat treatment necessary to achieve the best properties in iron-cobalt alloys, an investigation results. ll

. was made, of the alloys among others of the iron.- The above described and other objects and teacobalt group with vanadium. As a result of this tures of the invention will be apparent from the investigation it was found that alloys of iron, following detailed descrip en in onneccobalt and vanadium in quantities varying withtlon with the accompanying drawings, in which:

in definite percentage ranges and with definite Fig. 1 is a curve chart showing the results ob- I! heat treatments produced materials of high pertained with iron-cobalt in equal proportions conmanent magnet properties. These materlalshave taining vanadium in quantities varying from 7.4 unusual mechanical advantages; before hardenper cent to 16 per cent; ing for permanent magnet properties, a one- Fig. 2 is a curve chart showin e results 0bquarter inch round bar may be swaged down tained from alloys in which the qauntity 01' 86 from a three-quarter inch cast bar; may be bent vanadium employed a 10 P cent nd the cold without fracture; sheet forms may be rolled proportions of iron and cobalt vary; down to one-thousandth of an inch by one and Fig. 3 is a curve chart showing the variations a quarter inches wide without difliculty; the of remanence, coercive force and flux density 3 material thus produced has also been drawn into at a large magnetizing force for the optimum 80 fine wire. Specimens after quenching are easily composition for diflerent quenching temperadrilled, tapped and machined. A round onetures; quarter inch rod after quenching from 800 C. Fig. 4 is a curve chart showing a comparison had a Rockwellhardness (scale C) of 25, about between the optimum composition of the ap- 5 equal to that of soft steel. Final permanent m'agplicants' alloy and other permanent magnet 85 net properties obtained for the alloy by the materials; and present process are substantially the same for Fig. 5 is a composition triangle diagram showboth cast and worked material. These alloys ing the range of elements employed in the apthus have unequalled advantages in workability plicants alloys having highly desired magnetic 40 and machineability over permanent magnet properties.

materials heretofore known. Also they possess These alloys are in the same class magnetically definite advantages from a heat treating standas Honda steel (36 per cent '60, '7 per cent W, .4 point in that it is not necessary touse temperaper cent Cr, 0.6'l'per cent C, bal. Fe) and remalloy tures over 800 C. (72 per cent Fe, 12 per cent Co, and 16 per cent It was discovered that the alloys that proved Mo). An outstanding difference of the new advantageous from the standpoint of both peralloys from other permanent magnet materials is manent magnetic properties and machineability their mechanical properties; before they are ageshould have as their component elements 30 to 52 hardened they can be cold drawn and cold rolled per cent iron, 36 to 62 per cent cobalt and 6 to 16 almost as readily as soft steel.

per cent vanadium. The preferred composition The range 01' compositions comprising the sub- D the present invention is indicated by the elliptical area on the triangular diagram shown in Fig. 5. The extreme limits of this area are 30 and 52 per cent iron, 36 and 52 per cent cobalt and 6 and 16 per cent vanadium. The preferred composition, as stated above, so far has been found to be 38 per cent iron, 52 per cent cobalt and 10 per cent vanadium, which has a remanence of 9000 gauss and a coercive force oi 300 oersteds with a maximum energy product (BXH) of 102x10 The maximum energyproduct is usually taken as a figure of merit for permanent magnet material and a discussion of the maximum energy product may be had by referring to Chapter II of Applied Magnetism by T. F. Wall. The maximum (BXH) product is not usually determined because of the labor involved. Usually, the practice is to merely obtain the remanence Br and coercive force He. The product of BrXHc is higher than the maximum B H, but is roughly proportional to it. Generally, for the same BrXHc of two materials, the one with the higher Br is preferred. Hereinafter the product BrIIc is used as an index of the figure of merit.

Before hardening for permanent magnet properties, the new alloys are more workable and more easily fabricated than any other permanent magnet material heretofore known with comparable permanent magnet properties. They 'are easily machined, drilled or,cut and can be bent cold. The alloys have either in the worked or. cast condition about the same final permanent magnet properties after heat treatment so that they can be used either cast or worked, whicheversuits the purpose in mind. These materials, it merely baked at 600 C. after being worked, have very good permanent magnet properties. In one case the sample had about 10,315 gauss for remanence and 215 oersteds for coercive force. While this product (BrXHc) is not as high as for a complete heat treatment, namely a quench or an anneal followed by a bake, the remanence is high and the net result is very good for such a simple .heat treatment which does not require going above 600 C. The socalled baking treatment need not be given at a temperature above 600 C. but may be at a higher temperature up to about 800 C., the extreme limits of said ranges varying a little, but not much, with variations in the composition. Moreover, as the temperature goes up the time may be shortened until just below the critical point the time may bequite short but in no case should the temperature exceed a critical point around or Just below 800 C. In preparing strip material such as, for example, recording tape, it is awell known fact that a convenient and desirable method of heat treatment of strip material is given by using a continuous furnace through which the material is drawn. The fact that the material needs only a short time baking, if the temperature is right, is therefore a noteworthy advantage. A baking treatment applied to a cast specimen without an intermediate quench does not have so high a product, but

Finthem'lcre, connection with this baiting treatment, the dimension of the mass oi. rnaterial must be taken into consideration. Very thin specimens reach a uniform temperature throughout the mass almost instantly while larger bodies require more time. For this reason an exceedingly short treatment of a massive body at a higher temperature cannot be practically accomplished.

On account of this workability, it is particularly applicable for unusual shapes of magnetsv for magnets requiring mechanical precision and for purposes requiring material in thin or laminated forms. A commercial application for this material is for magnetic recording tape.

In many cases, permanent magnet structures are terminated by magnetic material of relatively higher permeability and larger flux carrying capacity than the permanent magnet in order to concentrate the flux from the permanent magnet across an air-gap. These new alloys are particularly adaptable as regards the heat treatment requirement of such terminating parts. In other words, the terminating parts may be welded to the permanent magnet and the whole assembly heat treated together. For example, if the permanent magnet has pole-pieces of iron, the whole assembly may be pot annealed at 900 or 10.00 C. which is a proper heat treatment for both the iron and the vanadium-ironccbal,t alloy. The second heat treatment in this case will be a bake at 600 C. from which temperature the whole assembly is cooled slowly. The second heat treatment makes the vanadium-ironcobalt alloy a permanent magnet and is not harmful to the. iron. These same remarks apply if an alloy of 45 per cent nickel, the balance iron, is used instead of the iron. If the terminating parts are an alloy of 78 per cent nickel, the balance iron, the whole assembly for the second heat treatment may be quenched from 600 C. after baking instead of cooled slowly. This produces the desired high permeability in the alloy of 78 /2 per cent nickel, the balance iron, and a rapid cooling from 600 C. is equally as good as slow cooling for the permanent magnet. The heat treatment of the assembly can also be adapted for pole-pieces composed of alloys of equal parts of iron and cobalt. "This adaptability is unique. It is often necessary with .other permanent magnet materials that parts have to be welded on the permanent magnet structure after heat treatment for the permarlent magnet is partly or wholly completed because the heat treatment of the permanent magnet is harmful to the magnetic properties of the terminating parts. In some cases this is only a partial remedy as the welding process still does a certain amount of harm to the magnetic properties of the terminating parts or the permanent magnets or both.

In the first period of this investigation vanadium-iron-cobalt alloys of equal proportions of iron and cobalt with varying vanadium contents up to 16 per cent were used. The samples tested included round cast bars three-quarter inch in diameter and rods one-quarter inch in diameter worked down from the three-quarter inch rods. Results of the same order were obtained for the same composition for both the worked and cast specimens. The specimens were first quenched in oil from 1200 to 1300 C. and then baked at 600 C. The magnetic results for this series of alloys are shown by curves in Fig. 1. The remanence, as seen from this curve chart cent.

decreases almost linearly with the percentage of vanadium, while the coercive force is almost proportional to the vanadium content up to a maximum of 280 oersteds at 14 per cent vanadium. A curve showing the variation of the product of remanence times coercive force (BrHe) is also shown. The product (BrHc) is roughly proportional tothe maximum energy product (BH). The product curve has its maximum value of 1.845 10 at 12 per cent vanadium corresponding to a remanence of 7500 gauss and a coercive force 01 246 oersteds. This diagram is conclusive evidence of a critical region of compositions.

The effect of varying the proportions of iron and cobalt was also investigated. Accordingly, six additional alloys were cast into three-quarter inch bars. The compositions of these alloys, their heat treatment and their magnetic properties for the cast specimens are given below in Table 1.

decrease in temperature.

tures from 800 C. to 1300 C. With the specimens in a quenched condition the maximum flux density and remanence are 18,800 and 5800 gauss, respectively, for a quenching temperature or 800 C. and remain practically constantfor quenching temperatures'up to 1100 0., and decrease slightly when quenched from 1200 and 1300 C. The coercive force is substantially constant at about oersteds. For the baked condition the maximum flux density and remanence are 16,200 and 9000 gauss, respectively, for quenching temperatures up to 1100 C. and fall of! slightly with further The coercive force has a value of 2'15 oersteds when quenched from 800 C. and increases with quenching temperatures to 295 oersteds for 1300 C. It is thus seen that (BrHc) is nearly constant for this range of quenching temperatures.

The results for quenching temperatures below Table I Perfif gg cent B. n, B.H. 1m Heat treatment 41 51 8 9570 200 l. 91 1300 0., 20 min., oil quench; baked 600 0., 18 hrs., in hydrogen. 51 41 8 8750 71 .62 1300 0., 20 min., oil quench; baked 600 0., 8 hrs., in hydrogen. 40. 5 49. 5 10 8850 282 2.50 1300 0., 20 min, oil quench; baked 600 0., 12 bra, in hydrogen. 49. 5 40. 5 10 8960 136 1. 22 1300 0., 20 min, oil quench; baked 600 0.,13 hrs., in hydrogen. 40 48 12 7350 318 2. 34 1300 0., 20 min., oil quench; baked 600 0., 12 hrs, in hydrogen. 48 40 12 7820 209 1. 64 1300 0.,20 min., oil quench; baked 600 0., 8 hrs., in hydrogen.

The results in the above table show that in the' I range covered for a given vanadium content the alloy with the larger amount of cobalt gave the best values. The alloy with a composition of 40.5 iron, 49.5 cobalt and 10 per cent vanadium had a (BrHc) of 2.5 10 This is the largest product for these six alloys.

As the next step in searching for an optimum composition for permanent magnet properties, a number of alloys were made in which the vanadium was held constant at 10 per cent and the proportions of ironland cobalt varied. The variations with per cent cobalt of remanence, coercive force, and (BrHc) products for these alloys are shown in Fig. 2. For the range of cobalt shown the remanence remains substantially constant at 9000 gauss with increase in cobalt up to 52 per With further increase in cobalt the remanence decreases in value. The coercive force increases almost linearly with increase in cobalt up to a maximum of 320 oersteds for 53 per cent cobalt. The maximum (BrHc) product is obtained for the alloy containing 52 per cent cobalt, 38 per cent iron and 10 per cent vanadium. The remanence is 9000 gauss and the coercive force 300 oersteds.

The heat treatment of the composition thus selected as an optimum was investigated more fully. Swaged specimens one-quarter inch in diameter were quenched from the following tem I peratures 600, 650, 700, 750, 800, 900, 1000, 1100, 1200 and 1300 C. These specimens were tested magnetically and then baked at 600 C. for various lengths of time before the second test. The results of these experiments are shown on Fig. 3. The variations with quenching temperatures of maximum flux density, remanence and coercive force corresponding to a maximum magnetizing force of 1735 oersteds, is shown for the specimens in both the quenched and baked conditions. For the baked condition the values of Br and Hr: associated with the highest (BrHc) product are given. A survey of the curves shown in Fig. 3 show the magnetic characteristics substantially constant for a wide range of quenching tempera- 800 C. are also interesting. With the quenching temperautre lowered to 750 C. the behavior of the alloys changed radically. The maximum flux densities and remanences decrease materially and almost coincide for the quenched and baked conditions. The curve for coercive force falls for the baked condition and rises for the specimens quenched so as" almost to meet. 0n the whole there is not much difference in the results of the two heat treatments.

As the quenching temperature is decreased below 750 C., the maximum flux density and remanence increase and the curves for the two heat treatments almost coincide. The resulting values are remarkably good for such low temperature treatments. The coercive force curves are similar in shape, but the values, as quenched, are lower than for the baked condition, the difference increasing as the quenching temperature is decreased.

X-ray examination showed the existence of only one phase in specimens that were quenched or slow-cooled from 800 C. or higher. Two phases appeared in these specimens after a subsequent baking at 750 C. Resistivity measurements on one-quarter inch rods corresponding to these conditions are given in Table II. (F. C. signifies furnace cooled.)

Table II Resistivity Heat treatment michmm.

Swaged from bar Pot annealed, 1000, 1 hr. F. C Pot annealed, 1000", 1 hr. F. 0.+750, F. 0... Swaged from 24 bar... 750,1hr.F.C

91.3 90.5 Rod No. 1 67.2

01.1 }liod No. 2

optimum. The results are shown in Table m. (P. A. signifies pot annealed.)

Table III 1 Heat treatment B, H, ;2 3:3

As cast 5040 64.6 As cast-+600 0., 8 1111's., linogycgoggaifsn 6500 296 A A t annea e .I. .l 5210 55.9 a," As casH-P. A. MW 0., 20 rs.

000 0.. 8 hIE., in hydrogen 8630 299 Pot annealed, 1,90g C.,t1 hiPF. 5035 51.6 34 8 ed from iame er fl hr., dlipd to 600, 4 hrs. F. C.-. 6160 58 M" A. l000 0., 1 hr., F. C.+O 0., 8 bra, inhydrogen 8100 290 $4 Swaged from diameter+600 0., 2111s.,

e '&i"'"56 6i5 Swage rom 4 me er zt c ztfii "ii ii-menu i 11.,0 queue '1 lira, in hydrogen 8750 305 4 As shown by this table substantial values can be obtained by baking after casting without an immediate quench. lhis table also shows that results of the same order can be obtained by substituting an anneal for quenching. As shown by the last result in the above table, baking at 575 C. produces about the same magnetic result as from 600 C. but requires about three times as long.

The curves shown in Fig. 4 compare the preferred new alloy with three competing permanent magnet materials, iron-cobalt-molyhdenum (72 Fe, 12 Co, 16 Mo) iron-cobalt-tungstenchromium-carbon (36 Co, 7 W, 4 Cr, 0.6 C, bal. Fe) and a nickel-iron-aluminum alloy (29 per cent Ni, 58 Fe, 13 Al). The new alloy is comparable in magnetic properties with the two alloys just mentioned. The maximum (BH) prodnot for the new alloy lies between that for the former and the latter of the two alloys to which reference has just been made. The iron-cobalttungsten-chromium-carbon alloy and the ironcobalt-molybdenum alloy have remanences of 9500 and 11,000 gauss and a coercive force of 240 and 250 oersteds, respectively, as compared with 9000 gauss and 300 oersteds for the new material. The mechanical properties of this new alloy are better than any existing permanent magnet material with useful magnetic properties.

. It was also found that by immersing the alloy, high in vanadium, in liquid air for several hours produced a product having satisfactory permanent magnet properties. For. example, a threequarter inch diameter bar of an alloyconsisting or 40.5 per cent iron, 45.5 per cent cobalt and 14 per cent vanadium was heated to 1200" C. for thirty minutes, quenched in oil. It was then immersed in liquid air for several hours and then baked at 800 C. for four hours. The specimen was measured after quenching and measured again after immersion in liquid air, and the increase in its magnetic properties due to the immersion in liquid air was very noticeable. The final results obtained after baking with this specimen were 5200 gauss for remanence, 329 oersteds for coercive force and 10,850 gauss for B at H=1735.

Fig. 5 shows the composition triangular diagram wherein the elliptical area marked thereon shows the ranges of ingredients in percentages that may be used to produce alloys more highly desirable for permanent magnet purposes. By

comparing this area with the percentages stated in the specification, the area shown in dotted lines on a chart in accordance with the percentages stated in the specification is trapezoidal ateaecr whereas the area or the actually preferred percentages is elliptical. This elliptical area has been drawn upon the basis of experiment but, of course, the utility or border line cases is a matter of judgment. However, because there is no practical manner of defining this elliptical area, it is intended that the appended claims setting forth ranges of competitions be construed to define essentially this elliptical area.

The vanadium, iron and cobalt alloy of the present invention may have small amounts oi other elements present without material detriment to the permanent magnet properties of the alloy. Any one of these elements may be preseat or added in the following quantities: 1 per cent silver, 0.1 per cent carbon, l. per cent coiumbium, 2 per cent chromium, 3 per cent copper, 1 per cent manganese,2per cent molybdenum, 0.25 per cent phosphorus, 0.5 per cent silicon, and 3 per cent tungsten. Magnets made in acct'irdance with the present invention are tough enough to withstand machining during the manufacturing stages; they are also adequately tough when completed to withstand the usual shock incidental to the handling and usage usually given to permanent magnets; they are perhaps not as tough as some other inferior permanent magnets previously used but on the other hand they are not nearly so brittle as many others which have been subject to breakagev upon dropping or similar rough treatment.

The term swaging is intended to include reduction in diameter or thickness of the material or a change in shape of the material in one or more operative steps by the application of mechanical force. Machining is intended to include tapping, drilling, cutting, filing or similar operations.

What is claimed is:

l. A process of manufacturing a permanent magnet article of high toughness and a product of coercive force in oersteds and remanence in gauss greater than which comprises treating a body of an alloy comprising 30 to 52 per cent iron, 36 to 62 per cent cobalt, and 6 to 16 per cent vanadium, minor constituents not being excluded, at 800 to 1300 C., thereafter cooling said material to considerably below 600 C. and thereafter maintaining said body around 600 C. for a period of hours and thereafter magnetizing the body to the required extent.

2. A process of manufacturing a permanent magnet article of high toughness and a product of coercive force in oersteds and remanence in gauss greater than 10 which comprises treating a body of an alloy comprising 38:1 per cent iron, 52:1 per cent cobalt, and 10:1 per cent vanadium, minor constituents not being excluded, at 800 to 1300 C., cooling said material substantially below 600 0., thereafter maintaining said body around 600' C. for a period of hours and thereafter magnetizing the body to the required extent.

3. A tough non-brittle magnetic article of high permanent magnet properties composed of an alloy comprising as essential constituents 30 to 52 per cent iron, 36 to 62 per cent cobalt, and 6 to 16 per cent vanadium produced by heating the alloy to from 800 to 1300 C., cooling substantially below 600 C., thereafter maintaining it at around 600 C. to 800 C. for a period of time from several hours at the lower range to a much shorter time at the higher range and thereafter magnetizing it.

4. A flexible strip of magnetic material of dimensions, mechanical properties and magnetic quality suitable for magnetic recording comprising iron between 30 and 52 per cent, cobalt between 36 and 62 per cent and vanadium between 6 and 16 per cent, heat treated at some stage of its manufacture at 800 C. to 1300 0., thereafter cooled to considerably below 600 C. and thereafter heated to between 600 C. and 800 C.

- 5. A composite magnetic structure comprising a'permanent magnet portion and a highly permeable magnetic pole-piece of low coercive force welded thereto, said permanent magnet portion comprising iron between 30 and 52 per cent, cobalt between 36 and 62 per cent and vanadium between 6 and 16 per cent, said structure having been treated as a unit after assembly by heating to between 800 C. and 1300 C.', cooled substantially below 600 C., and thereafter maintained for a substantial period of time at around 600 0. whereby said permanent magnet portion is given a high capability of retaining magnetism and said pole-piece is given the desired permeability and low coercive force, and magnetizing the permanent magnet portion.

GEORGE A. ELSALL. ETHAN A. NESBITT. 

